Westercons, Worldcons, and other bidded conventions like Costume-Con hold site selection elections, and you can vote on site. The actual box into which you drop the ballots has been any sort of material. Commonly we just get a handy cardboard box. In Winnipeg in 1994, one of the committee members borrowed a very nice metal ballot box from his local union hall.
Costume-Con 26 very generously gave me a little bit of budget money to buy a new ballot box, because the one that jbriggs has had on semi-permanent loan to west coast bidded conventions appears to have gone walkabout. I ended up ordering this one.
I dithered for quite a while on the wisdom of having a clear plastic box for ballots, but in the end concluded that having people be able to see that votes (and nothing but votes) are in the box is a good thing. I also expect it will make people attend to folding their ballots more carefully. And we may manage to avoid the surprisingly common case of someone putting their receipt in the ballot box and walking away with their ballot. (This has happened at least once in nearly every site selection in which I've been involved.)
I've now offered the use of this box to this year's Westercon and Worldcon site selection administrators, inasmuch as I expect to drive to both Las Vegas and Denver, so transporting the box to where it needs to go shouldn't be a problem. (I can use it to pack stuff while I'm traveling, so it won't take appreciable extra space in transit.)
Big thanks to Costume-Con 26 for funding a bit of fannish infrastructure that (assuming we act responsibly) can be used for a long time to come.